Protective layering process for circuit boards

ABSTRACT

A polymer layering process that encapsulates and protects electronics components with complex and imprecise geometries. The protective layering process provides a combination of a flexible mold and/or a rigid mold that apply close-forming, encapsulating the polymer layers to the electronic components and precision assemblies. Polymer layer protective jackets are shaped to as-populated circuit boards and assemblies, providing tightly fit barriers with fine resolution accommodating imprecise geometries. The protective jackets can be formed in rigid, semi-rigid, or highly flexible polymer films, to protect the circuitry from the elements, CTE mismatches, shock and vibration loads and extreme g-forces.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 USC §119(e) of U.S.provisional patent application 61/563,939, filed on Nov. 28, 2011, whichis incorporated by reference in its entirety.

GOVERNMENTAL INTEREST

The invention described herein may be manufactured and used by, or forthe Government of the United States for governmental purposes withoutthe payment of any royalties thereon.

FIELD OF THE INVENTION

The present invention relates in general to the field of circuit boardmanufacturing. More specifically, this invention relates to an improved,protective, thin film layering process that protects electroniccomponents with complex and imprecise geometries. The protectivelayering process provides a combination of a flexible mold and a rigidmold, or another means to produce the same, that apply close-forming,encapsulating polymer or other layers to the electronic components.

BACKGROUND OF THE INVENTION

Conformal coatings are widely used in both the military and industrialelectronics applications, for protecting circuit board assemblies frommoisture, dust, chemicals, and temperature extremes, to prevent damageor failure of the electronic components. While the use of conformalcoatings offers several advantages compared to uncoated circuit boardassemblies, their application constitutes a “wet-process” which requiresthe use of hazardous chemicals that must be applied by spraying,brushing, or dipping, followed by drying and/or curing processes.

In addition, it would be difficult to control the conformal coatingthickness as well as the formation of pin-holes. With the exception ofparylene, which must be applied by expensive vacuum-deposition equipmentand which does not lend itself to high-volume production, most organicconformal coatings are readily penetrated by water molecules.

For a conformal coating to be effective, ionizable contaminants, such assalts, must be prevented from reaching the circuit nodes where they cancombine with water to form microscopically thin electrolyte layers thatcan be both corrosive and electrically conductive. Also, for theconformal coatings to adhere properly to the circuit board assemblies,thereby minimizing peeling, de-wetting, and the propensity to formpin-holes, all surface contamination must be removed prior to theapplication of the conformal coating, using another “wet-process” suchas a vapor degreasing or semi-aqueous washing in a special equipment.Special shielding and masking measures must also be taken while applyingthe conformal coatings to prevent it from contaminating connectors,sensitive components and the circuit board assemblies.

The application of a close-fitting, thin-layer of polymer, or anothermaterial in flat-sheet form, over the circuit board assembly and itselectronic components, either by a vacuum or pressure molding, or byother suitable processes, would offer superior protection from moisture,dust, chemicals, and temperature extremes compared to conformalcoatings. A thin polymer layer, or multiple thin layers, could beselected to provide various additional attributes, such as, improvedheat dissipation, ESD and EMI protection and control, and protectionfrom handling and in-use shocks.

Thin polymer layers could be added to the circuit board assemblies foruse in non-potted as well as potted applications. In pottedapplications, the polymer layers would offer additional benefits such asforming a barrier to prevent the potting material from seeping intoareas around and underneath sensitive components. After being cured,potting materials could cause high stresses, such as a residual stressand a thermal expansion stress, during temperature cycling, due to thecoefficient of thermal expansion mismatches and also due to contractionand expansion of the potting material itself.

More specifically, potting materials are being used with increasingfrequency, in both commercial and military applications, to encapsulatethe electronic components and circuit board assemblies of electronicsystems. The use of potting materials allows for a simplersupport-structure (while also enabling a smaller over-all system design)as well as enhanced structural support for the electronic components andcircuit board assemblies against shock and vibration.

A major disadvantage with encapsulants or potting materials however, isthe fact that they are permanent solid bodies that prevent any access orservicing of the components they encapsulate. Potting materials arealmost always thermoset materials that harden once and cannot not bere-softened or reused

In numerous military munition designs, where the electronic componentsmust survive the extremely high g-forces experienced during gun-launch,the potted electronics are inactive until the munition is used. Untilthis time the munition may have been in storage without environmental(temperature and humidity) controls for up to 20 years.

In contrast, the electronics for most commercial applications tend to beactive for most of their lifetime where the operating environment ismore stable and predictable. Without external temperature controls, orthe fairly constant temperature environment that active electronicscreate for themselves, inactive electronic components experiencecontinuously varying physical stresses which are created due to theirintimate contact with the potting material and the different rates ofexpansion and contraction that each produces with changes intemperature. If the changes in temperature are severe enough, orrepeated a sufficient number of times, the physical stresses induced onthe inactive electronic components can be severe. The resultant loads orstresses can be high enough to fracture the ceramic lids ofhollow-cavity devices, or other types of electronic components, and mayalso lift components completely off of their circuit boards.

In addition, during the potting process, the potting material may seepinto the open spaces between the leads of the chips and also underneaththe chip packaging. The potting material that has seeped into theseareas will create residual stresses in the solder joints and alsoagainst the packaging bottom surface after the potting material hassolidified during the curing process.

Currently, the following failures have been observed for pottedelectronics during either the temperature-cycling qualification processor the life test (temperature-cycling and gun-launch) of a sub-system ofthe fielded-artillery system:

-   -   1. Solder joints failed during the temperature-cycling process.    -   2. Solder joints failed during the life test.    -   3. Lids and lid-seals cracked on MEMS (e.g.        MEMS—Micro-Electro-Mechanical Systems) open-cavity devices        during the temperature-cycling process and also during the life        test.    -   4. Tiny electronic devices pulled off from their circuit boards        during the temperature-cycling process.

The application of a barrier, such as a thin layer of polymer (or othermaterial), over the electronic components prior to the addition of thepotting material, is believed necessary to help mitigate the abovefailures. The polymer layer can be applied by various processes such asheat, vacuum, vacuum plug assist, radio frequency forming or acombination of processes, or a combination of the above.

For failures resulting from the solder joints failure and componentsbeing pulled off their circuit board assemblies during temperaturecycling, the polymer layer would prevent the potting material fromintruding between the chip-leads and also under the chips, and thus helpprevent the push and pull stresses that the potting material wouldproduce as it expands and contracts with increasing/decreasingtemperatures.

For failure resulting from both lids and lid-seals being cracked onhollow-cavity devices, the polymer layer would provide: 1) alow-adhesion boundary between the potting material and the lid surfacesthereby mitigating the high shear-stresses that would develop as thepotting material expands and contracts due to ambient temperaturefluctuation, and 2) a compliant layer that would minimize thehigh-compression stresses that the potting material develops when itexpands, due to increasing temperatures, and high-tension stresses thatthe potting material develops when it shrinks, due to decreasingtemperatures, against the lids of these devices.

What is therefore needed is a process of forming and emplacing the thinpolymer, or other formable composite, layers so as to precisely conformto the imprecise geometries of the electronic components on the circuitboard assemblies, despite the imprecise geometries of these componentsdue to their geometrical tolerance, placement tolerance as well as themanufacturing and assembly variances of the circuit board assemblies. Itwould also be substantially advantageous that the polymer layers besufficiently strong to provide the structural support of the potting tothe circuit board assemblies during high-g force events. It wouldfurther be desirable to have the thin polymer layers be sufficientlyflexible to allow for differentials in coefficients of thermal expansionbetween the circuit board assemblies and the potting material.

Certain publications, such as U.S. Pat. No. 5,318,855, propose a methodto vacuum form a polymer film over the circuit board assemblies toprovide electrical and environmental protection. However, the proposedmethod does not seem to allow the polymer layer to precisely conform tothe electronic components.

U.S. Pat. Nos. 4,959,752 and 4,768,286 suggest vacuum forming polymerlayers over a circuit board assembly to closely conform to the geometryof the circuit board assembly prior to the application of the pottingmaterial. However, these layers must be thin enough to permit vacuumforming over the circuit board assemblies, and are therefore too thin toprovide sufficient structural support or to provide sufficient boundaryto differential thermal expansion.

Another process of forming multiple layer films into packages to protectprinted circuit boards is described in U.S. Pat. No. 7,161,092. Thispatent generally describes a method of forming a plurality of layers tocover the approximate shape of a printed circuit board assembly asopposed to mounting the electronic components in a container orenclosure. This method describes the bonding of at least three layers(i.e., insulating, conductive, and abrasion protection) into a conformalfilm that can be stamped or pressed, and then adhered to the electroniccomponent assemblies, which may require breather valves. The surfacetension of the individual layers usually provides an approximate fit,that is a fit with substantial radii or a loose fit encapsulatingtechnique.

What is therefore needed is a process of forming the layers without theneed for bonding individual layers together, and that produces a verytight fit between the polymer layers and the circuit board assembly. Inaddition, desirable process would not require adhesives or meltingoperations to join the layers to the circuit board assembly, and wouldprovide an exacting polymer layer fit that facilitates snapping of thepolymer layer to the assembly it is designed to protect. Such adesirable process would provide a polymer layer that follows thecontours (or profiles) of all the electronic components on the circuitboard assembly and would allow for variances in dimensional tolerances,placement and assembly. Prior to the advent of the present invention,the need for such a protective layering process has heretofore remainedunsatisfied.

SUMMARY OF THE INVENTION

The present invention satisfies this need, and describes an improvedprotective layering process that protects electronics components withcomplex and imprecise geometries. The protective layering process uses acombination of a flexible mold and/or a rigid mold that applyclose-forming, encapsulating polymer layers (or other composite layerscreated from other formable materials) to the electronic components.

The imprecise geometries are due to the normal electronic componentgeometrical tolerances as well as position-variations inherent in themanufacturing processes of the circuit board assemblies. It is anobjective of the present invention to provide molding tools and formingmethodologies that apply equal pressure, which will make the protectivelayers tightly fit to the electronic components of the circuit boardassemblies, even perpendicular to the direction of molding. The softdurometer of the flexible mold face will flow or deform aroundindividual components, applying force on sides of components in additionto the projected or top surfaces of components. This dual-material molddesign is ideally suited to form polymer layers or other formablecomposite over electronic devices with varying tolerances.

To this end, the present invention includes a method for encapsulating acircuit board assembly for enhanced survivability in harsh and extremeenvironments with a thin polymer layer (or other composited layerscreated from other formable materials) tightly fit to the possibleimprecise, as-built geometries of a populated, circuit board assembly.By precision scanning a circuit board assembly, and using the deriveddata, a male and/or a female mold can be created which is asubstantially exact representation of the geometries of a completedcircuit board assembly.

The data may be adjusted to scale the pattern, and therefore theassociated mold, to allow for layer thicknesses, to form multiplesuccessively larger layers, which may include thermally conductive,electromagnetic shielding layers, layers for impact protection,environmental protection, which may be placed on top of the first layer,to account for associated mold shrinkage, production polymer layer (orother formable composite layers) shrinkage, or for other reasons.

The polymer layer may be heated and a vacuum may be drawn through themold, pulling the heated layer into a substantially preciserepresentation of the circuit board assembly. The mold may also beheated, which allows for better temperature control of the thin polymerlayer as it is formed, as opposed to an unheated mold, or as opposed toforming the layer directly against the circuit board assembly.

This forming (or molding) process allows for a substantially preciselayer or layers to be produced using a broad range of materials,including such materials that may not be formed directly against thecircuit board assembly. Formed layers may be applied to the circuitboard assembly by physical emplacement, with the aid of heat and/or airpressure, with a precisely formed rigid mold or with the aid of aconformal mold or by a combination of rigid and conformal molds.

The conformal mold may have a rigid backing or reinforcement layer(heated or unheated) and a soft, front layer which substantially matchesthe various geometries of the circuit board assembly. When pressure isapplied and the soft, conformal mold comes in contact with polymercovered circuit board assembly components, and is forced to flow ordeform around individual components, the mold applies force on the sidesof components as well as to the projected or top surfaces of theelectronic components. The result of compressing the conformal mold overthe polymer layer and the circuit board assembly is a protective layerthat is tightly fitted to all components.

This method of encapsulating the circuit board assembly is particularlyadvantageous to create a barrier layer between the circuit boardassembly and the potting compound which may be applied on top of thethin layer. As stated earlier, the polymer layer is precisely formed tothe imprecise, as-built geometries of the circuit board assembly.

The polymer layer may be fabricated to be both flexible enough toprotect the circuit board assembly from damage caused by differentialsin coefficients of thermal expansion, due to environment temperaturefluctuations, between the circuit board assembly and the pottingmaterial, without any appreciable degradation in the structural supportprovided to the circuit board assembly by the potting materials duringextreme, high-g force events.

The advantages of the present invention include but are not limited to:

-   -   Provide a barrier for all lead frames, chip carrier frames,        miniature electronic devices, wired devices and all solder        joints from direct contact with the potting material, which will        then reduce the stresses caused by the expansion, contraction,        and lifting-action caused by the potting material during changes        in temperature.    -   Provide a flexible stress-reducing medium between the electronic        components of the circuit board assembly and the potting        material during changes in temperature and also during high        shock/vibration events.    -   Eliminate the shear forces experienced by the electronic        components, due to adherence of the potting material, during        changes in temperature and also during high shock/vibration        events.    -   Create a barrier that prevents the potting material from flowing        between the electrical leads and also underneath the electronic        packaging/components, thereby preventing the potting material        from pushing/pulling the components off the circuit board during        changes in temperature.    -   Provide a moisture barrier between the electronic components and        the operating environment.    -   Provide EMI shielding both from nearby internal electronic        components and also from EMI sources external to the electronic        components.    -   Provide a low-adhesion interface between the electronic        components and the potting material that will allow for easier        dissection/serviceability of the circuit board assembly.    -   Provide a thermal dissipation medium for high-heat generating        electronic components.    -   Provide one or more of the following to the circuit board        assembly: chemical protection, corrosion protection,        contamination protection, salt spray protection, fungus        protection, dust protection.

These and other advantages and features of the present invention will bemore readily understood from the following detailed description ofpreferred embodiments of the invention that is provided in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention and the manner ofattaining them, will become apparent, and the invention itself will bebest understood, by reference to the following description and theaccompanying drawings, wherein:

FIG. 1 is a perspective view of an exemplary circuit board assembly tobe protectively layered according to present invention;

FIG. 2 is a high level flow chart of a layering process, which includesa mold making phase and a polymer layer forming phase according to thepresent invention;

FIG. 3 is a more detailed flow chart of the mold making phase of thelayering process of FIG. 2;

FIGS. 4 through 9 are illustrations of the various steps of the moldmaking phase of the layering process of FIG. 3;

FIG. 10 is a more detailed flow chart of the polymer layer forming phaseof the layering process of FIG. 2, according to a first embodiment ofthe present invention;

FIGS. 11 through 14 are illustrations of the various steps of thepolymer layer forming phase of the layering process of FIG. 10;

FIG. 15 is a more detailed flow chart of the polymer layer forming phaseof the layering process of FIG. 2, according to a second embodiment ofthe present invention;

FIGS. 16 through 18 are illustrations of the various steps of thepolymer layer forming phase of the layering process of FIG. 15;

FIG. 19 is a more detailed flow chart of the polymer layer forming phaseof the layering process of FIG. 2, according to a third embodiment ofthe present invention;

FIGS. 20 through 23 are illustrations of the various steps of thepolymer layer forming phase of the layering process of FIG. 19;

FIG. 24 is a more detailed flow chart of a conformal mold making phaseof the layering process of FIG. 2;

FIGS. 25 through 30 are illustrations of the various steps of theconformal mold making phase of FIG. 24;

FIG. 31 shows the polymer layer forming phase of the layering process,using the conformal mold made using the mold making phase of FIGS. 24through 30;

FIG. 32 is an illustration of another embodiment of the conformal moldof FIGS. 24 through 31; and

FIG. 33 is an illustration of still another embodiment of the conformalmold of FIGS. 24 through 31.

Similar numerals refer to similar elements in the drawings. It should beunderstood that the sizes of the different components in the figures arenot necessarily in exact proportion or to scale, and are shown forvisual clarity and for the purpose of explanation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates an exemplary circuit board assembly 100 which is tobe protectively layered according to present invention, in order toprotect electronics components 101, 102, 103, 104 with complex andimprecise geometries, that are secured (e.g., soldered) to a circuitboard 111. To this end, the present invention includes a protectivelayering process 200 that is generally illustrated in FIG. 2.

The process 200 generally comprises two phases: a mold making phase 300(as further illustrated in FIGS. 3-9 and 24-30), and a polymer layerforming phase 1000 (1500 or 1900, as further illustrated in FIGS. 10-23and 31-33). These phases 300 and 1000 (1500 and 1900) will now bedescribed in greater detail.

FIG. 3 provides a more detailed flow chart of the mold making phase 300of the layering process 200 of FIG. 2. An aspect of forming the polymerlayers according to the present invention, for the circuit boardassembly 100, is to provide polymer layers with a substantially exactfit to the circuit board assembly 100. To create a mold that duplicatesthe circuit board assembly 100 exactly, it is determined that a moldcast from a populated circuit board assembly 100 would be superior to amachined mold. A machined mold would need to have been made from a CADfile that might not include all the imperfections or imprecisegeometries of the actual populated circuit board assembly 100. It isimportant that the process 200 would allow for the shrinkage in both themold casting process (e.g., aluminum) and the thermoplastic formingphase 1000 (FIG. 10) of the actual polymer layers.

At step 305 of FIG. 3, and as further illustrated in FIG. 4, the maincircuit board assembly 100 to be protected is separated from auxiliaryboards and connections.

At step 310 and as further illustrated in FIG. 5, the circuit boardassembly 100 is scanned. A point cloud file 500 is then created from thescanned image. A white light laser computer tomography device (CT scan)may be used to trace the intricate physical geometries of the circuitboard assembly 100, and to record all its features in the electronicpoint cloud file 500. The point cloud scan file (geometric points) isconverted into a data or surface file by grouping the points intosurfaces which are also blended or stitched together to form a singlecomputer aided design/CAD file.

At step 315, the data/surface file is enlarged to another surface file600 (FIG. 6), in order to accommodate for shrinkages of pattern(s),mold(s) and the protective films that will be deposited at a later stageof the phase 1000 (FIG. 10). In an exemplary embodiment, thedata/surface file 500 was enlarged (i.e., grown) by approximately 0.012″in to accommodate for the shrinkage of the cast aluminum mold(approximately 0.009″ in) and also for the shrinkage of the plasticfilms (approximately 0.003″).

At step 320, the enlarged surface file 600 may be converted to a STEP orlayer file to facilitate printing of the enlarged model by a SLA/stereolithography machine or other rapid prototyping or model printingmachines.

At step 325, a negative or cavity plaster mold 700 is created. FIG. 7illustrates one such plaster mold 700. More specifically, in theexemplary embodiment, stereo lithography pattern was used to create theplaster mold 700. A core box, with sprue and runner, was added to theplaster negative mold 700 to enable positive metal casting using theplaster cavity mold. The plaster negative 700 was oven dried after itwas cast.

At step 330, a positive male metal mold 800 is made from the plastercavity mold 700. The male mold 800 is also referred to as a drape mold.FIG. 8 illustrates one such metal mold 800. In the present exemplaryembodiment, molten aluminum was poured into the plaster negative mold700 to make the aluminum positive mold 800. The aluminum positive mold800 is wire brushed and cleaned.

At step 335, and as illustrated in FIG. 9, several holes, i.e., 910,920, 930 are drilled in the aluminum positive mold 800 of FIG. 8, so themold is capable of plastic forming. The holes 910, 920, 930 will enablevacuum suction of the polymer films that will be deposited according tophase 1000 of FIG. 10. In the exemplary embodiment, the aluminum mold800 is wire drilled/electrical discharge machined (EDM) to provide theholes 910, 920, 930 for pulling a vacuum or vacuum forming of the thinpolymer films.

One or more polymer layers can now be applied to the circuit boardassembly 100 by forming the thin layers on a drape mold (also called amale mold) or a cavity mold (also called a female mold). The precise fitof the layers to the circuit board assembly 100 can be formed onmetallic molds that are duplicate copies of the circuit board assembly100. The accuracy of the mold is a result of the scanning and printingprocess described earlier, which provides a mold geometry that is moreaccurate than machining and which includes even the smallest of featuresand textures. Using a metallic mold enables precise control of heatingand cooling that was not possible when using circuit board assemblies orprinted wiring boards as forms or molding tools.

With reference to FIG. 10, it illustrates the polymer layer formingphase 1000 of the layering process 200 of FIG. 2, according to a firstembodiment of the present invention. The phase 1000 summarizes theprocess of forming one or multiple polymer layers that are also referredto as protective jackets to the circuit board assembly 100 with vacuum.

The phase 1000 enables a manufacturer to furnish thin film, polymerlayers for the supplied circuit board assembly 100 in a number ofthermoplastic materials. The polymer layers act as a barrier for avariety of potting materials used to encapsulate the circuit boardassembly 100, to fill undesirable voids, and to adapt to discrepanciesin the geometries of the electronic components of the circuit boardassembly 100.

At step 1005 of the polymer layer forming phase 1000 of FIG. 10, a thinthermoplastic sheet 1100 (FIG. 11) is clamped (or affixed) to a rigidframe of the forming machine (not shown). While the thermoplastic sheet1100 is still affixed to the rigid frame, it is heated at step 1010under quartz lamps, until it softens. The thermoplastic sheet 1100 willsag when heated providing a visual that the thermoplastic sheet 1100 isready to be formed. The heating temperature of the thermoplastic sheet1100 varies with the material used. As an example, surface temperaturesmay vary between approximately 200° F. and 240° F. The heating step 1010may for example take between approximately 20 to 40 seconds.

At step 1015, the thermoplastic sheet 1100 is moved (or lowered) overthe drape or male mold 900. A machine slide enables the preciseplacement of the thermoplastic sheet 1100 film over the mold 900. Asfurther illustrated in FIG. 11, the heated and softened thermoplasticsheet 1100 is lowered over the mold 900 and pushed down past the mold900, to ensure a tight fit to the geometries of the electroniccomponents of the circuit board assembly 100, as replicated on the malemold 900.

At step 1020, vacuum pressure of, for example, approximately 31 lbs.gauge is applied through the vacuum holes 910, 920, 930, to pull thethermoplastic sheet 1100 further around all the intricate features ofthe electronic components of the circuit board assembly 100.

At step 1025, the thermoplastic sheet 1100 is allowed to cool on themold 900 using fan-forced, room temperature air. The cooling step 1025would take approximately 20 seconds. The frame of the molding machinecontaining the cooled, formed thermoplastic sheet (or envelope) 1200(FIG. 12) is raised.

At step 1030, the thermoplastic envelope 1200 is released from theframe, and any excess material of the thermoplastic sheet 1100 trimmed.

At step 1035, the thermoplastic envelope 1200 is placed on the circuitboard assembly 100 (FIG. 13), and secured thereto. The enveloped circuitboard assembly 100 is shown in FIGS. 13 and 14 and is designated by thenumeral 1300.

The polymer layer forming phase 1000 may be repeated to form multiplelayers on the circuit board assembly 100. The polymer layer formingphase 1000 offers a method of applying thin polymer layers over theelectronic components of circuit board assemblies, which fit accuratelyaround all components. The thin polymer layer(s) will have uniformthickness and consistent quality across the entire part as required bythe application.

With reference to FIG. 15, it illustrates a polymer layer forming phase1500 of the layering process 200 of FIG. 2, according to a secondembodiment of the present invention. The phase 1500 summarizes theprocess of forming one or multiple polymer layers that are also referredto as protective jackets to the circuit board assembly 100 with vacuum.

At step 1505, a thin thermoplastic sheet 1100 (FIG. 16) is clamped (oraffixed) to a rigid frame of the forming machine (not shown). While thethermoplastic sheet 1100 is still affixed to the rigid frame, it isheated at step 1510 under quartz lamps, until it softens. As indicatedearlier, the thermoplastic sheet 1100 will sag when heated, providing avisual that the thermoplastic sheet 1100 is ready to be formed. Theheating temperature of the thermoplastic sheet 1100 varies with thematerial used. As an example, surface temperatures may vary betweenapproximately 200° F. and 240° F. The heating step 1010 may for exampletake between approximately 20 to 40 seconds.

At step 1515, the thermoplastic sheet 1100 is moved (or lowered) overthe drape or male mold 900. A machine slide enables the preciseplacement of the thermoplastic sheet 1100 film over the mold 900.

At step 1515, and as further illustrated in FIG. 16, the heated andsoftened thermoplastic sheet 1100 is lowered over the mold 900 andpushed down past the mold 900, to ensure a tight fit to the geometriesof the electronic components of the circuit board assembly 100, asreplicated on the male mold 900.

At step 1520, vacuum pressure of, for example, approximately 31 lbs.gauge is applied through the vacuum holes 910, 920, 930, to pull thethermoplastic sheet 1100 further around all the intricate features ofthe electronic components of the circuit board assembly 100. This stepresults in a preformed thermoplastic envelope (or layer) 1710 (FIG. 17).

At step 1525, a heated negative mold 1700 is pressed against thepreformed thermoplastic envelope 1710 and the mold 900 to furtherincrease resolution (fitment) including undercuts. The negative mold1700 is then removed, and the thermoplastic envelope 1710 is allowed tocool on the mold 900 using fan-forced, room temperature air at step1530. The cooling step 1525 would take approximately 20 seconds.

At step 1530, the thermoplastic envelope 1710 is released from the mold900, and any excess material of the thermoplastic sheet 1710 trimmed.

At step 1535, the thermoplastic envelope 1710 is placed on the circuitboard assembly 100 (FIG. 18), and secured thereto, by for example, alayer of adhesive that can be applied to either the thermoplasticenvelope 1710 or the circuit board assembly 100 (or both). The envelopedcircuit board assembly 100 is shown in FIG. 18 and is designated by thenumeral 1800. The polymer layer forming phase 1000 may be repeated toform multiple layers on the circuit board assembly 100.

With reference to FIG. 19, it illustrates a polymer layer forming phase1900 of the layering process 200 of FIG. 2, according to a thirdembodiment of the present invention. The phase 1900 summarizes theprocess of forming one or multiple polymer layers to the circuit boardassembly 100 with pressure.

With further reference to FIGS. 20 through 23, the polymer layer formingphase 1900 provides an alternative method of forming a thermoplastic orpolymer layer 1100 using a heated female tool or negative cavity mold1700 that is oriented to face in the downward position, facing thepolymer layer 1100. The negative mold 1700 includes heat or air channelsthat are switchable from pulling vacuum to applying air pressure.

Heated molds are sometimes oriented facing downward (also referred to asupside down). The main reason for this orientation is that the conveyorfeed where the items, such as circuit board assemblies, can be droppedinto nests and remain in their position under the effect of gravity.Once in the forming machine, the circuit board assembly 100 is raised upinto the cavities for over-molding, dropped back down on the conveyorand the cycle is repeated. Most often the male or female molds are on awork surface facing upward as vacuum is normally mounted below themachine. In addition, molds lower on a surface and facing upward providea line of sight into the mold so that the operators can monitor thecomplete process.

The topography of the negative mold 1700 includes cavities and smallfeatures 2000, 2010 (FIG. 20) that correspond to the geometries of thecomponents of the positive mold 900 and the circuit board assembly 100.The phase 1900 can form polymer layers in a single operation, or it canbe used as a secondary operation to press-fit polymer layers (formed bythe other forming methods, such as methods 1000 or 1500) over existingcircuit board assemblies or printed circuit boards.

One aspect of the polymer layer forming phase 1900 is to have layers ofvarious thicknesses and polymer composition be superposed and fittightly over the electronic components of the circuit board assembly100. By forming the layers to the geometry of the electronicscomponents, a barrier is created to separate the circuit board assembly100 from potting materials. Potting materials are used to encapsulatethe sensitive electronics and cast them into a solid body.

The polymer layer barriers prevent potting materials from flowingunderneath small electronic chips or components where they can be liftedoff the printed circuit boards due to thermal movement among thedissimilar materials. The polymer barrier layers further protect theelectronic components during manufacturing, assembly, and further duringthe storage life and product use from potting liquids, moisture,contaminants, dust and corrosion.

At step 1905, the heated thermoplastic sheet 1100 is applied to thenegative mold 1700 (FIG. 20). At step 1910, vacuum pressure is used topull the heated thermoplastic sheet 1100 tightly into the cavities andsmall features 2000, 2010 of the negative mold 1700, in order to producea preformed thermoplastic envelope 2100 (FIG. 21).

The heated thermoplastic sheet 1100 conforms very accurately to theshape, placement, and geometry of the electronic components. As aresult, the present phase 1900 addresses problems associated with theunderside and vertical surfaces of the thermoplastic sheet 1100 and howclosely if covers each electronic component. In order to accurately formthe underside and internal sides of the thermoplastic sheet 1100, adrape or male mold is used in addition to the female mold (also referredto as cavity mold or appearance mold). A female mold may be used ifsurfaces of the mold need to be machined to produce larger parts. Femaletools are considered “steel safe” because sections of the mold may beremoved to accommodate larger electronic components.

At step 1915, and while the preformed thermoplastic envelope 2100 isstill positioned at least in part, within the negative mold 1700, thepositive Mold 900 (or alternatively the circuit board assembly 100), isthen moved into the cavities 2000, 2010 of the negative mold 2100, insuch a way as to sandwich the thermoplastic envelope 2100 therebetween.This step will ensure that the thermoplastic envelope 2100 completelyencases the geometries of components that are being protected.

While the thermoplastic envelope 2100 is still warm, the positive mold900 is clamped in place to the negative mold 1700. The air direction inthe negative mold 1700 is then reversed to press the thermoplasticenvelope 2100 tightly over the geometries of the clamped positive mold900.

At step 1920, the thermoplastic envelope 2100 is then allowed to cooland to shrink over the components geometries of the positive mold 900(that correspond to the electronics components of the circuit boardassembly 100). The thermoplastic envelope 2100 is cooled to a specifictemperature in order to hold its shape and to prevent distortion.

At step 1925, and as illustrated in FIG. 22, the thermoplastic envelope2010 is released from the mold 900, and any excess material of thethermoplastic envelope 2010 is trimmed. As an example, the outside edgesof the thermoplastic envelope 2100 are trimmed to the shape of thecircuit board assembly 100. Temperature sensitive tape may be employedto bond the edges of the thermoplastic envelope 2100 shape to the flatof the circuit board 111 (FIG. 1).

Alternate methods of bonding the thermoplastic envelope 2100 barrier tothe circuit board assembly 100 include adhesive bonding, solvent bondingto a texture, and ultrasonic welding to textures or three-dimensionalfeatures on the circuit board assembly 100. Also, an undercut, doublebend or perimeter snap can be utilized to attach the thermoplasticenvelope 2100 to the circuit board assembly 100 similarly to hardwareblister packaging or food containers.

At step 1930, the thermoplastic envelope 2100 is placed on the circuitboard assembly 100 (FIG. 23), and secured thereto, by for example, alayer of adhesive that can be applied to either the thermoplasticenvelope 2100 or the circuit board assembly 100 (or both). The envelopedcircuit board assembly 100 is shown in FIG. 23 and is designated by thenumeral 2300. The polymer layer forming phase 1900 may be repeated toform multiple layers on the circuit board assembly 100.

The phase 1900 is also applicable to solve potential circuit boardassembly electromagnetic interference (EMI) problems, and may includethe following steps:

1) Application of an insulation layer to the components, section orassembly requiring protection. The layers will be tightly fit aroundcomponents.

2) Application of an electrically conductive shielding product to thetop of the first layer. These materials include metal foils, metalizedfabrics or cloth, metal particle polymer composites, plated fibercomposites, vacuum metalized layers, electroless plated layers, etc.

3) Solder or mechanically attach the shielding material to the groundfeatures on the circuit board assembly 100.

In situations where the electronic devices of the circuit board assembly100 have very high heat dissipation/generation, the following step willbe added prior to the application of the electrically conductiveshielding product to the top of the first layer:

Cut the non-conductive layers around the periphery of the hot electroniccomponent, and remove the non-conductive cutout. Replace the removednon-conductive layers with high thermal conductive material or pad totransfer heat away from the electronic component.

FIG. 24 illustrates a conformal mold making phase of the layeringprocess 200 of FIG. 2, which is referenced by the numeral 2400.According to this phase 2400, a conformal mold 2800 is fabricated with aflexible (soft or conformal) layer 2700 that is secured to a rigid layer(or backing) 2710 that enables the application of close-forming,encapsulating polymer layers (or layers) to the circuit board assembly100 having complex and imprecise electronic component geometries. Theimprecise geometries of the electronic components of the circuit boardassembly 100 may be due to the normal electronic component geometricaltolerances as well as position-variations inherent in the circuit boardassembly 100 manufacturing processes.

It is an objective of the conformal mold 2800 to provide a molding toolthat applies equal pressure to the electronic components of the circuitboard assembly 100, including surfaces that are perpendicular to thedirection of forming (or molding). The soft (or flexible) durometer ofthe flexible (e.g., compliant) mold face will allow the polymer to flowor deform around individual electronic components, and the applicationof a force on the sides of the electronic components as well as to theprojected or top surfaces of the electronic components. This dual-layer(soft-rigid) mold design is suited to form polymer layers overelectronic devices with varying tolerances.

While soft presses are known to conform polymer layers to the circuitboard assembly 100, as in U.S. Pat. No. 7,752,751, they have not been ofsubstantially flat nature and have not been shaped to substantiallymatch the shape of the circuit board assembly 100. Therefore, they donot apply pressure as uniformly to the polymer layer as the presentinvention.

The present invention includes creating the conformal or flexible face2700 on a molding tool that will apply close-forming polymer layers onthe circuit board assembly 100. The geometric variations of the circuitboard assembly 100 include, for example: component-placement positioningvariations; solder paste height variations; geometrical tolerances ofthe electronic components; and open-cavity devices (e.g., MEMs) sealthickness variations. With all these variations, the conformal mold 2800of the present invention will provide consistent and accurateapplication of encapsulating polymer layers (or layers) over theindividual electronic components.

It is an object of the phase 2400 to fabricate a conformal mold 2800with a soft section 2700 whose shape is developed based on thecombinations of the nominal dimensions of the electronic components ofthe circuit board assembly 100, the electronic components geometricaltolerances, and the electronic components placement variations. Theconformal mold 2800 can be primarily constructed as a silicone orelastomer soft body which may be backed up by a rigid support plate2710, or may be a shaped silicone or elastomer face on a rigid moldwhich follows the geometry of the circuit board assembly 100.

Referring now to FIG. 24, it illustrates the conformal dual-layer moldmaking phase of the layering process 300 of FIG. 2, which is alsoreferred to as phase 2400 to distinguish this phase over the other moldmaking phases described herein.

At step 2405, and with further reference to FIG. 25, the actual,populated circuit board assembly 100 (or an exact replica thereof, i.e.,the positive mold 900) is placed in a casing or fixture 2510 thatprovides for accurate positioning and orientation of the circuit boardassembly 100.

At step 2410, and with further reference to FIG. 26, in order to createthe dual-layer mold 2800, a pattern of the final circuit board assembly100 topography is created by applying successive, patterned polymerlayers 2600 to the circuit board assembly 100. The polymer layers 2600are of appropriate thicknesses and flexibility so that they closely formto the topology of the electronic components. The successive layers 2600can be applied, for example, with heat and vacuum from below (or aheated dual-layer mold), and air pressure from above the circuit boardassembly 100. Soft pads may be used to apply pressure over electroniccomponents of the circuit board assembly 100. The successive polymerlayers 2600 account for the actual thermoplastic sheets or layers thatwill formed at the later stage of the manufacturing process.

At step 2415, and with further reference to FIG. 27, in order to preparea cavity and shape 2750 for the soft layer 2700 of the mold 2800, thenumber of successive polymer layers 2600 needs to be determined. Layercount and layer thickness are dependent on several factors, among whichare the following: the requirements of the final encapsulated circuitboard assembly 100 including environmental, physical and mechanicalconsiderations; and the desired thickness of the soft layer 2700 toprovide a robust forming tool while satisfying the service lifeexpectancy. In creating the soft layer 2700, the thickness of thepolymer encapsulation layers 2600 needs to compensate for the electroniccomponents dimensional tolerances. Designing and creating the conformalmold (or soft) face 2700 to accommodate for component tolerancerequirements ensures a constant interface pressure between the softlayer face 2700 and the polymer layers 2600 despite the geometriccomplexities of the electronic components.

At step 2420, and with further reference to FIG. 28, the rigidreinforcement portion 2710 of the conformal mold 2800 is made using thefixture 2510 and the circuit board assembly 100 with the applied polymerlayers 2600 and spacing 2750. The conformal or soft portion 2700 of themold requires a high degree of details from the topography of thecircuit board assembly 100, such as small surface mount devices,component leads, white-wires, etc. However, the rigid portion 2710 ofthe mold 2800 serves to strengthen the conformal face 2700.

As such, the rigid portion 2710 of the mold 2800 can be made fromcasting materials, usually thermoset resins with metal fillers, as theback or reinforcement is not expected to experience high stress or wear,nor excessively-high temperatures, while in use. The rigid portion 2710of the mold 2800 may have features such as dovetails or keyways 3200(FIG. 32) to mechanically engage the flexible material (soft layer) 2700when poured, creating a mechanical bond between the two dissimilarmaterials. The casting is performed in such a way that it locatesaccurately with the fixture 2510 holding the circuit board assembly 100.

At step 2425, and with further reference to FIG. 29, the soft compliantportion 2700 of the conformal mold 2800 is formed by placing anotheractual circuit board assembly 100 or mold 900 into the fixture 2510, andby applying a very thin polymer layer 2900 as described earlier. Thispolymer layer 2900 is made as thin as possible to allow for thepossibility that the electronic components sizes and positions on thecircuit board assembly 100 may have ranged to their largest tolerances.

Next, a mold release is applied to the polymer layer 2900. In creatingthe soft mold face 2700, silicone (or another suitable elastomericmaterial with appropriate properties) is applied to thecomponent-cavities 2750 (FIGS. 27, 28) and other surfaces that will facethe circuit board assembly 100. During the pouring or injection of theflexible material for the soft mold face 2700, the silicone orequivalent material will flow into the locking features of the rigidmold backing 2710, bonding them as a single unit. Next, the combinedmold 2800 is allowed to cure around the fixtured circuit board assembly100. The height of the compliant mold 2800 relative to the circuit boardassembly 100 must be controlled so that the flexible mold face thicknessis maintained with regard to the circuit board assembly 100 and theelectronic components thereon.

At step 2430, and with further reference to FIG. 30, once the flexiblemold face material (soft layer 2700) has cured, the fixture (or moldingtool) 2510 can be separated from the circuit board assembly 100. Thecompliant mold 2800, which includes both the soft layer 2700 and therigid backing 2710, is now ready to be used to apply the encapsulationpolymer layer(s) to the circuit board assemblies 100 with complex andimprecise geometries.

FIG. 31 illustrates the process of applying one or more polymer layersto the circuit board assembly 100. In this process, a circuit boardassembly 100 from a product line with imprecise geometries is ready forapplying a close-forming polymer layer (or layers) 1100 or 1710 to itssurface. The conformal mold 2800 is used to apply pressure to thepolymer layer 1100 or 1710, as described earlier in connection withprocesses 1000 (FIG. 10), 1500 (FIG. 15), and 1900 (FIG. 19). Asillustrated by the arrows in dotted lines, in some instances, it mightnot be possible or advantageous to apply vacuum from underneath thecircuit board assembly.

If the height of the electronic components exceeds formability of aselected polymer layer, the circuit board assembly 100 could optionallybe encapsulated by a polymer layer with cutouts around the tallcomponent sections. Pre-formed polymer covers can be used to wrap aroundthe tall components and joining procedures such as adhesives, solventbonding, ultrasonic welding, or radio frequency sealing will ensure atight seal between the circuit board assembly 100 polymer layer and thesmaller polymer covers for the tall components.

The soft conformal mold face (or layer) 2700 may be made of a singlematerial construction or multiple layers of different materials.

The conformal mold 2800 provides the ability to form or process polymerlayers to a great degree of accuracy, resolution, and tolerance. Whilethe forming methods 1000, 1500, and 1900 use heat, vacuum, and/orpressure, the conformal mold 2800 uses mechanical forming around theintricate components. The soft or conformal face 2700 of the conformalmold 2800 has the ability to mechanically form or shape thethermoplastic layers in addition to the vacuum or air pressure forming.The soft face 2700 of the mold 2800 is soft enough to flow around theelectronic components when closed against the opposite mold or pressedtightly onto the populated circuit board assembly 100. In this manner,the soft geometry of the compliant mold face 2700 pushes the polymerlayer not only on the top surface of components, but it also appliespressure to the layer on the sides of components as the compliant moldface 2700 deforms to comply with the geometries of the electroniccomponents.

In addition, the present three-dimensional mechanical forming produceslayers with a significant fit to the unique topology of the numerouscircuit board assemblies despite minor differences on the height, axialand position tolerance of soldered components.

FIG. 33 illustrates a compliant mold 3300 according to anotherembodiment of the present invention. The soft compliant layer 2700 isformed according to process 2400, as described earlier, and is shownenlarged relative to its actual size for the purpose of illustration.Conductive traces 3310 are formed within the soft compliant layer 2700in order to facilitate the heating of the soft compliant layer 2700.

In this particular exemplary embodiment, the conductive traces 3310include thermally conductive fillers 3320, connecting wires 3333, andembedded heating pads 3330. One or more externally accessible pads 3344may also be added.

While it is highly desirable to cause the conformal mold 2800 to formpolymer layers around the electronic components and complex geometries,the soft compliant face 2700 that has an internal heat source (heatsources, or conductive traces) 3310 can form the parts faster and withgreater accuracy. Elastomers and silicones are thermally non-conductiveallowing very little heat to pass through the material even in thincross sections.

It is an objective of the invention to create conformal or soft moldsthat are actually thermally conductive to soften thermoplastic layermaterial quickly. Using gentle heat from inside the soft mold 2700, thepolymer layer materials are kept in the softened state until pressurefrom the mold completes the forming or net-shape. Air cooling, orcooling systems inside the forming machine, lower part temperatures to“freeze” the part so it can be handled and used immediately.

Exemplary mold heaters include for example, a silicone pad type whilecartridge heaters, wire networks, ceramic elements and piped fluids arealternate heating methods.

To create heat transfer from the heating elements inside the conformalmold 3300 to the polymer layers, a thermally conductive mold materialwas developed. Thermally conductive fillers 3320 can be added to thesilicone or elastomer mold materials while keeping the molds at Shore Adurometers that range from 60-80. The fillers 3320 are small in size sothe mold 3300 does not become too rigid and loose the conformalqualities.

Exemplary fillers include aluminum powder, copper platelets, alumina andboron particles, ceramic whiskers, metal-plated graphite fibers,tungsten fibers, and carbon black. Various ratios of filler to moldresin could be used, depending on the circuit board assembly 100.

The following exemplary filler composition using aluminum fillers tomake the compliant molds 3300 resulted in the following:

15% filler provided minimal heat transfer;

25% filler provided 20-40% heat transfer;

37% filler provided 45-60% heat transfer; and

50% filler provided 65-75% heat transfer.

The compliant molds 2800 and 3300 allow a uniform thickness andconsistent quality of polymer layers 1100 to be applied to the surfaceof the circuit board assembly 100 with complex and imprecise geometries.They also provide the numerous advantages among which are the following:

The compliant face 2700 of the mold 2800 enables forces in the X (leftand right) and Y (front and back) directions, in addition to the Z (upand down) or the directions perpendicular to the draw.

The interface pressures between the compliant face 2700 and surfaces ofall the components are substantially consistent.

The compliant face 2700 of the mold 2800 will adapt to the shape of theelectronic components when it is pressed against the circuit boardassembly 100 and the polymer layers 1100, and even allows the forming ofpolymers to minor undercuts;

The compliant face of the mold 2800 reduces the need for heavy draftused to release the encapsulated circuit board assembly 100 from themolding tool.

The compliant material of the mold 2800 can be cast with internal airchannels or voids that enable the soft mold face 2700 to expand, toaccommodate additional deformation, and to and stretch for compliantmolding of intricate geometries and geometries with a high depth-to-drawratio.

The processes of forming the polymer layers according to the presentinvention can be done using a combination of films (sometimes called capstock) in a laminate or one layer formed on top of another. Multiplepolymer layers can be designed to solve a variety of load and harshenvironmental problems. The following examples are provided forillustration purpose:

Example A Polymer Layer with EMC (Electro Mechanical Compatibility)Protection

The present process is suited to form a non-conductive layer followed byan electrically conductive layer to shield the circuit board assembly100 against unintended electrical transmission or infiltration. Theelectrically conductive or shielding layer may utilize nickel platedfibers, metallic fibers, conductive veils or cloth, electro less nickelplating of the thermoplastic layer or vacuum deposition of metalparticles on the polymer layer.

Example B Insulative Polymer Layers

A protective barrier for extreme cold is readily accomplished using thepresent invention. First, a non-conductive layer is formed, followed bya “foamed” layer or a layer that has a cellular structure. The airbubbles or cells from the density reduction act as an effectiveinsulator. Wires or pads internal to the polymer layers can function asinstant heaters for cold start-up. Reference is made to FIG. 33.

Example C Heat Extractor Polymer Layers

Plastics offer very little conduction of heat. So many circuit boardassemblies rely on metal surfaces to radiate heat away from electricaland electronic components while other assemblies use mechanical movementof air or forced convection. There will be cases where the polymerlayers will be formed (or molded) with air channels and/or wickingstructures for heat pipes or heat extractors. Metal pads over componentsserve as thermal pick-ups which then pipe the heat outside of theenclosed area or to outside air.

The following is a non exclusive list of exemplary polymers that can beused to form the thermoplastic sheets or layers: polycarbonate,polyethylene, siloxane rubber, alloy grade with added styrene,polyolefin materials, low-density polyethylene, linear low-densitypolyethylene, high-density polyethylene, polypropylene, metalocene basedpolyethylene, polyvinyl chloride, and high impact polystyrene.

It should be understood that other modifications may be made to thepresent embodiments without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A method for encapsulating a populated circuitboard assembly having an imprecise three-dimensional geometry, with atleast one protective layer that is tightly fit to the imprecise geometryof the populated circuit board assembly, the method comprising: making amold of the populated circuit board assembly; using the mold toencapsulate the populated circuit board assembly with said at least oneprotective layer; wherein making the mold includes: scanning thepopulated circuit board assembly to create a surface file of thepopulated circuit board assembly; enlarging the surface file toaccommodate for variations of the mold and said at least one protectivelayer; using the enlarged surface file to create a positive mold;wherein encapsulating the populated circuit board assembly with said atleast one protective layer includes: using the positive mold to create amulti-layer compliant mold; heating a first protective layer of said atleast one protective layer; drawing the heated first protective layerwithin the compliant mold, in order to pre-form an envelope with asubstantially precise representation of the populated circuit boardassembly; pressing the positive mold against the pre-formed envelopewithin the compliant mold to form the envelope; releasing the formedenvelope from the positive mold and the compliant mold; and placing theformed envelope onto the populated circuit board assembly to beencapsulated.
 2. The method according to claim 1, further comprisingcreating at least one additional protective layer atop said at least oneprotective layer.
 3. The method according to claim 1, wherein creatingthe multi-layer compliant mold includes creating a rigid reinforcementlayer.
 4. The method according to claim 3, wherein creating themulti-layer compliant mold further includes creating a soft compliantlayer that deforms to comply with the three-dimensional geometry of thepopulated circuit board assembly.
 5. The method according to claim 4,wherein the soft compliant layer includes a heating element tofacilitate heating the soft compliant layer.
 6. The method according toclaim 5, wherein the heating element includes any one or more of: aconductive trace, aluminum powder, copper platelets, alumina and boronparticles, ceramic whiskers, metal-plated graphite fibers, tungstenfibers, and carbon black.
 7. The method according to claim 4, furthercomprising securing the soft compliant layer to the rigid reinforcementlayer.
 8. The method according to claim 7, wherein securing the softcompliant layer to the rigid reinforcement layer includes engaging thesoft compliant layer to the rigid reinforcement layer by means of atleast one dovetail pattern.
 9. A method for encapsulating a populatedcircuit board assembly having an imprecise three-dimensional geometry,with at least one protective layer that is tightly fit to the imprecisegeometry of the populated circuit board assembly, the method comprising:making a mold of the populated circuit board assembly; using the mold toencapsulate the populated circuit board assembly with said at least oneprotective layer; wherein making the mold includes: scanning thepopulated circuit board assembly to create a surface file of thepopulated circuit board assembly; enlarging the surface file toaccommodate for variations of the mold and said at least one protectivelayer; using the enlarged surface file to create a positive mold;wherein encapsulating the populated circuit board assembly with said atleast one protective layer includes: using the positive mold to create amulti-layer compliant mold; heating a first protective layer of said atleast one protective layer; drawing the heated first protective layerwithin the compliant mold, in order to pre-form an envelope with asubstantially precise representation of the populated circuit boardassembly; placing the positive mold against the pre-formed envelopewithin the compliant mold to form the envelope; and placing the formedenvelope onto the populated circuit board assembly to be encapsulated.